Typically, during a normal operation of a washing machine, a user loads articles to be cleansed into a washer basket, selects a wash cycle, and starts the machine. The washing machine then performs a number of operations to complete the wash cycle. Generally, the wash cycle includes a wash operation, a spin operation, a rinse operation and a spin operation. The wash operation includes filling the washer basket and a washer tub which contains the basket with water to a user selected level. An agitator disposed in the washer basket then imparts an oscillatory motion to the water and detergent (wash liquid) and the articles. The oscillatory motion causes the articles and wash liquid to move back and forth in the washer basket. This movement provides mechanical energy which is used to assist in removing soils from the articles. After agitating the articles and wash liquid for a predetermined length of time, the liquid is then pumped out of the washer basket and washer tub. Generally, this is followed by a spin operation to reduce the remaining wash liquid. The rinse operation is similar to the wash operation in that it includes filling the washer basket and the washer tub to a previously assigned level, agitating for a predetermined amount of time, and pumping the wash liquid out of the basket and tub. Typically, the wash cycle includes one wash operation and one rinse operation, but most washing machines provide an optional extra rinse operation to further remove any remaining detergent. Once a majority of the wash liquid has been removed by the rinse operation, the spin operation is activated to extract additional liquid from the articles. During the spin operation, the washer basket rotates in one direction at a high angular velocity. This rotation creates a centrifugal force on the articles and the wash liquid causing excess liquid to exit or be extracted through perforations in the washer basket wall.
In order for the wash cycle to effectively clean the articles, it is necessary to ensure that the washer basket and washer tub are filled with an adequate amount of water for agitation. If the amount of water provided is too low, then the articles might not have enough water to effectively clean the articles. In addition, too low of a water level will result in a large amount of mechanical stress on the agitator and its drive system. Furthermore, if there is a low level of water, then the articles cannot move as well which increases the possibility of damage to the articles. On the other hand, if too much water is added, then some of the articles will float in the washer basket and not receive enough interfacial wash action from the agitator to effectively clean the articles. Too much water is also energy inefficient because water is being wasted along with energy expended to heat, pump, and agitate the extra water. Another problem with adding too much water is that the agitator will not be able to impart the proper amount of back and forth motion to the articles for optimal cleaning or rinsing.
One approach that has been used to overcome the above problems is to automatically control the amount of water added to the washer basket and washer tub during a wash cycle. This approach generally uses the weight of the articles to be cleansed as a factor in determining the amount of water to be added. Determining the weight of the articles has been achieved by typically measuring the torque on the agitator and drive system and then determining the inertia of the articles in the washer basket. Measuring the torque on the agitator and drive system and determining the inertia of the articles requires complex and expensive equipment. Using weight sensors to determine the weight of the articles is another approach that has been used. However, weight sensors are expensive and have to be continually calibrated. Accordingly, there is a need to be able to determine the weight of the articles without having to rely on complex and expensive equipment so that the amount of water added to the washer basket and washer tub during a wash cycle can be automatically controlled.